This invention pertains generally to nickel-base superalloys useful in the manufacture of hot-section components of aircraft gas turbine engines, e.g., vanes and rotating blades, and more particularly to yttrium and ytrrium-silicon bearing compatible coatings especially useful for the enhancement of the environmental resistance of such hot-section components made from advanced nickel-base superalloys and nickel-base eutectic superalloys.
Vanes and rotating blades cast conventionally from nickel-base superalloys typically consist of equiaxed nonoriented grains. Recognizing the effects of grain boundaries on high temperature mechanical properties, much effort has been expended to improve the properties of such vanes and blades by strengthening the grain boundaries through the addition of grain boundary strengtheners, such as boron and zirconium, elimination of grain boundaries transverse to the major stress axis, or elimination of grain boundaries altogether.
By the use of directional solidification (DS) as is described, for example, in U.S. Pat. No. 4,202,400, which is incorporated herein by reference, it is possible to produce parts such as vanes and rotating blades having an oriented microstructure of columnar grains whose major axis is parallel to the major stress axis of the parts and which have few or no grain boundaries perpendicular to the major stress axis. A further advance has been to use directional solidification techniques to produce vanes and rotating blades as single crystals, thus eliminating high angle grain boundaries and orienting low angle grain boundaries parallel to the major stress axis while minimizing the presence of low angle grain boundaries.
Yet another advance in materials for high temperature gas turbines are the advanced nickel-base eutectic superalloys such as the monocarbide reinforced nickel-base eutectic superalloys of the type described, for example, in U.S. Pat. 4,292,076 to Gigliotti, Jr. et al., which is incorporated herein by reference. The superalloys of U.S. Pat. No. 4,292,076, when directionally solidified under stringent conditions to achieve planar front solidification, result in a eutectic composite microstructure consisting of strong, reinforcing metallic carbide (MC) fibers in a .gamma./.gamma.' nickel-base superalloy matrix. Because highly aligned anisotropic microstructures are formed during planar front solidification, the superalloys of U.S. Pat. No. 4,292,076 offer potential structural stability and property retention to a greater fraction of their solidification temperatures than do other materials.
In order to take full temperature advantage of the advanced nickel-base superalloys and nickel-base eutectic superalloys, however, coatings are required to provide environmental protection at the high intended use temperatures. Stringent requirements are placed on the coatings and the coating/substrate composite. For example, the coatings must be tightly bonded, i.e., metallurgically bonded, to the substrate and ideally must not degrade either the mechanical properties of the substrate (e.g., ductility, stress rupture strength and resistance to thermal fatigue) or the chemical properties of the substrate (e.g., oxidation resistance and hot corrosion resistance).
Examples of adverse effects to eutectic superalloys which have resulted from prior art incompatible coatings are fiber denudation near the coating/substrate interface due to outward diffusion of carbon from the fibers into the coating and the formation of brittle precipitates, generally in the form of needle-like platelets, in the substrate due to interdiffusion of elements between the coating and the substrate. Similarly, zones denuded of the gamma prime (.gamma.') strengthening phase and formation of brittle precipitates have been observed in single crystal nickel-base superalloys due to the use of incompatible coatings.
While many coatings and barrier/coating systems have been proposed and tried, there has been a general inability in the past to specify coatings or barrier/coating systems which are truly compatible with advanced nickel-base superalloy and nickel-base eutectic superalloy substrates, i.e., offer improved environmental protection and produce good metallurgical bonds with the substrate yet not degrade the mechanical or chemical properties of the substrate.
Therefore, there exists a need for protective environmental coatings which are truly compatible with the newest generation of nickel-base superalloys and nickel-base eutectic superalloys, particularly those designed for use as vanes and rotating blades in aircraft gas turbine engines.